Scala 中 def 和 val 的区别 [英] Difference between def and val in Scala

问题描述

循环定义如下:

 def loop : Boolean = loop

当 x 定义为:def x = loop 时，控制台会显示x: Boolean".

When x is defined as: def x = loop then "x: Boolean" is shown in console.

和

当 x 定义为:val x = loop 则进入无限循环

When x is defined as: val x = loop then it goes to infinite loop

我知道 def 使用的是按名称调用，而 val 使用的是按值调用.尽管上面定义的关于循环的这一点不是很清楚.

I know def is using call by-name and val is using call by-value. Even though this point about loop defined above is not much clear.

推荐答案

def 不评估作业的右侧.就像

def doesn't evaluate the right-hand side of the assignment. Just like

def f(x : Int) = x + 2

不(在这种情况下逻辑上不能)评估 f 的任何内容，只是定义函数 f 的含义，既不是 def 循环:Boolean = loop 或 def x = loop 评估任何东西.您只是定义了一个要在其他时间执行的函数.

doesn't (in this case logically can't) evaluate f of anything, just defines what the function f, means, neither def loop : Boolean = loop nor def x = loop evaluate anything. You are just defining a function to be executed at some other point.

但是 val 确实需要对赋值的右侧进行评估.所以 val x = loop 尝试执行右侧的表达式.但是，尝试计算 loop 永远不会终止，因为 loop 是一个无限循环.

But vals do require the right-hand side of the assignment to be evaluated. So val x = loop tries to execute the expression on the right-hand side. Trying to evaluate loop never terminates, though, since loop is an infinite loop.

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